The invention relates to compositions and methods useful in the acceleration of binding of target analytes to capture ligands on surfaces. Detection proceeds through the use of an electron transfer moiety (ETM) that is associated with the target analyte, either directly or indirectly, to allow electronic detection of the ETM.
There are a number of assays and sensors for the detection of the presence and/or concentration of specific substances in fluids and gases. Many of these rely on specific ligand/antiligand reactions as the mechanism of detection. That is, pairs of substances (i.e. the binding pairs or ligand/antiligands) are known to bind to each other, while binding little or not at all to other substances. This has been the focus of a number of techniques that utilize these binding pairs for the detection of the complexes. These generally are done by labelling one component of the complex in some way, so as to make the entire complex detectable, using, for example, radioisotopes, fluorescent and other optically active molecules, enzymes, etc.
Other assays rely on electronic signals for detection. Of particular interest are biosensors. At least two types of biosensors are known; enzyme-based or metabolic biosensors and binding or bioaffinity sensors. See for example U.S. Pat. Nos. 4,713,347; 5,192,507; 4,920,047; 3,873,267; and references disclosed therein. While some of these known sensors use alternating current (AC) techniques, these techniques are generally limited to the detection of differences in bulk (or dielectric) impedance.
The use of electrophoresis in microfluidic methods to facilitate the binding of biological molecules to their binding partners for subsequent detection is known; see for example U.S. Pat. Nos. 5,605,662 and 5,632,957, and references disclosed therein.
Similarly, electronic detection of nucleic acids using electrodes is also known; see for example U.S. Pat. Nos. 5,591,578; 5,824,473; 5,705,348; 5,780,234 and 5,770,369; U.S. Ser. No. 08/911,589 now U.S. Pat. No. 6,232,062; and WO 98/20162; PCT/US98/12430; PCT/US98/12082; PCT/US99/10104; PCT/US99/01705, and PCT/US99/01703.
One of the significant hurdles in biosensor applications is the rate at which the target analyte binds to the surface for detection and the affinity for the surface. There are a number of techniques that have been developed in nucleic acid applications to either accelerate the rate of binding, or to concentrate the sample at the detection surface. These include precipitation of nucleic acids (see EP 0 229 442 A1, including the addition of detergents (see Pontius et al., PNAS USA 88:8237 (1991)); partitioning of nucleic acids in liquid two phase systems (see Albertsson et al., Biochimica et Biophysica Acta 103:1-12 (1965), Kohne et al., Biochem. 16(24):5329 (1977), and Mxc3xcller, Partitioning of Nucleic Acids, Ch. 7 in Partitioning in Aqueous Two-Phase Systems, Academic Press, 1985)), as well as partitioning in the presence of macroligands (see Mxc3xcller et al., Anal. Biochem. 118:269 (1981)); and the addition of nucleic acid binding proteins (see Pontius et al., PNAS USA 87:8403 (1990) and U.S. Pat. No. 5,015,569), all of which are expressly incorporated by reference. In addition, partitioning systems for some proteins have also been developed, see Gineitis et al., Anal. Biochem. 139:400 (1984), also incorporated by reference.
However, there is a need for a system that combines acceleration of binding of target analytes, including nucleic acids, to a detection electrode for subsequent electronic detection.
In accordance with the objects outlined above, the present invention provides methods of detecting a target analyte in a sample. The methods comprise concentrating the target analyte in a detection chamber comprising a detection electrode comprising a covalently attached capture ligand. The target analyte is bound to the capture ligand to form an assay complex comprising at least one electron transfer moiety (ETM). The presence of the ETM is then detected using the detection electrode.
In a further aspect, the concentration step comprises placing the sample in an electric field between at least a first electrode and at least a second electrode sufficient to cause electrophoretic transport of the sample to the detection electrode.
In an additional aspect, the concentration step comprises including at least one volume exclusion agent in the detection chamber.
In a further aspect, the concentration step comprises comprises precipitating the target analyte.
In an additional aspect, the concentration step comprises including at least two reagents that form two separable solution phases, such that the target analyte concentrates in one of the phases.
In a further aspect, the concentration step comprises binding the target analyte to a shuttle particle.
In an additional aspect, the invention provides methods of detecting target analytes comprising flowing the sample past a detection electrode comprising a covalently attached capture ligand under conditions that result in the formation of an assay complex. As above, the assay complex further comprises at least one electron transfer moiety (ETM), and the presence of the ETM is detected using said detection electrode.
In a further aspect, the methods are for the detection of target nucleic acids and include the use of hybridization accelerators. The assay complex is formed in the presence of a hybridization accelerator, that may be a nucleic acid binding protein or a polyvalent ion.
In an additional aspect, the invention provides methods of detecting a target analyte in a sample comprising adding the sample to a detection electrode comprising a covalently attached capture ligand under conditions that result in the formation of an assay complex. The conditions include the presence of mixing particles.
In a further aspect, the invention provides substrates comprising a plurality of gold electrodes. Each gold electrode comprises a self-assembled monolayer, a capture ligand, and an interconnect such that each electrode is independently addressable. Preferred substrates include printed circuit board materials such as fiberglass.
In an additional aspect, the invention provides methods of making a substrate comprising a plurality of gold electrodes. The methods comprise coating an adhesion metal onto a fiberglass substrate, and coating gold onto the adhesion metal. A pattern is then formed using lithography, and the pattern comprises the plurality of electrodes and associated interconnects. The methods optionally include adding a self-assembled monolayer (SAM) to each electrode.
In an additional aspect, the invention provides methods of making a substrate comprising a plurality of gold electrodes. The methods comprise coating an adhesion metal onto a substrate, and coating gold onto the adhesion metal. A pattern is then formed using lithography, and the pattern comprises the plurality of electrodes and associated interconnects. The methods further include adding a self-assembled monolayer (SAM) to each electrode.